This project utilizes a synapse between a large axon and an interneuron in the spinal cord of the lamprey, a primitive vertebrate, to examine the role of Ca ions in transmitter release. The ability to position micro-electrodes both pre- and post-synaptically under visual control, and the accessibility of a discrete synapse for close impalement presynaptically, make this an attractive preparation for several types of experiments. 1) For example, the tetrodotoxin-blocked synapse will be used to describe the interaction between the external Ca ions concentration and the level of presynaptic depolarization in determining such features of the synaptic transfer curve as the "threshold" presynaptic depolarization and the maximum post-synaptic potential. 2) Manipulation of the external Ca ions will also permit the power function relating transmitter release to the external Ca ions concentration to be determined, the first such observation at a central synapse. 3) Other experiments will explore the possibility that presynaptic depolarization, in addition to increasing Ca ions conductance, also alters the interaction between Ca ions and a site on the presynaptic membrane. Because this alteration would be expected to change the normal power function, any such effect of presynaptic depolarization can be determined at the tetrodotoxin-blocked synapse by comparing the power functions obtained for several different levels of presynaptic depolarization. 4) To determine whether the Ca ions site is internal or external, cation ionophores will be used to move Ca ions into the fiber by means other than through the normal Ca ions channels. This will alter the normal power function if the site is external but will be without effect if the site is internal. 5) The cation ionophores will also be utilized to determine if the normal rate of release of transmitter can be enhanced through pharmacological manipulation. 6) Finally, a series of cations will be injected into the presynaptic fiber in the region of the synapse to study the antagonism between Ca ions and certain cations. These inotophoresis experiments will also permit an analysis of whether internal Ca ions affects transmitter release through a kinetic or a screening action.